The present invention relates to a method for the surface treatment of a tribological coating made of a supereutectic aluminum-silicon alloy and an aluminum-silicon laminate with a coating structure wherein the surface is reworked after the coating is produced.
Methods for the treatment of supereutectic aluminum-silicon alloys are known in themselves. These alloys serve especially for the production of workpieces with surfaces of wear-resistant, low-friction tribological coatings. Such workpieces and coatings are used, for example, in automobile manufacture to produce internal friction surfaces in crankcases and cylinder liners.
Present-day light construction crankcases for piston machines consist, for cost reasons, of subeutectic aluminum-silicon alloys which are made by pressure casting. This material, however, does not provide satisfactory friction and wear qualities. Cylinder liners or at least their interior friction surfaces must therefore consist of a wear-resistant, low-friction, tribological material.
DE 44 38 550 A1 has disclosed a cylinder liner made from a supereutectic aluminum-silicon alloy which has fine silicon primary crystals and intermetallic phases in the form of hard particles. Such a material must then still be surface-treated: first a fine boring is performed, and then the surface is smoothed by honing. This takes place in series production in at least two working steps which are called xe2x80x9cpre-honingxe2x80x9d and xe2x80x9cfinish honing.xe2x80x9d In a final step the silicon particles contained in the alloy, which form the actual friction surface, are exposed by etching the aluminum away with an aqueous solution of an acid.
EP 0 565 742 A1 has disclosed a honing process for refining workpiece surfaces in at least two steps. In one of the process steps the surface of the workpiece is finish-honed to the final dimension. Thus a very finely honed texture is produced in the surface. In another process step, which can be performed before or after the finish honing, striations intersecting one another are produced by a radiation apparatus, especially a laser. The final surface in this case has both honing striations and laser radiation traces.
Another possibility consists in coating the internal friction surfaces of the cylinder liners after the crankcase has been cast. This is accomplished, for example, by plasma spraying as described in DE 195 08 687 C2. By this method a layer of iron or steel alloy can be applied which is characterized by satisfactory friction and wear qualities.
German patent applications 197 33 204.8-45 and 197 33 205.6-45, which have an earlier priority but are not yet published, filed on Aug. 1, 1997, a hot-sprayed coating of a supereutectic aluminum-silicon alloy and aluminum-silicon composite is disclosed which is characterized by a heterogenic coating structure of a solid solution of aluminum, a coarse to very fine network of eutectic silicon, silicon segregations and particles, intermetallic phases and extremely finely divided oxides. This coating contains characteristic primary aluminum solid solution dendrites in which the dendrite arms are enveloped in eutectic silicon. The photomicrographs of such coatings show a characteristic sponge-like appearance. Silicon primary segregations and silicon particles are present only in a small percentage and have a small diameter. In the surface treatment of these coatings, the dendrite arms at the surface are lightly ground, so that in the exposure that follows the aluminum is etched away and the aluminum-free silicon structures remain, which form the actual friction surface.
The surface treatment of supereutectic aluminum-silicon coatings, regardless of their composition and structure, is nevertheless very complicated. Lubricants must be used which then in some cases must first be completely removed again before the hard particles are exposed.
The present invention is therefore addressed to the problem of devising a method of the kind referred to above which will be less complicated and less costly.
The solution is to finish the surface by machining it dry, without lubricant, in a one-step procedure, using a cutting tool with at least a diamond-containing cutting material.
The idea of the invention thus consists in replacing the conventional, complicated wet treatment such as honing, for example, with a dry finishing process in which a cutter with at least a diamond-containing cutting material is used. Surprisingly it was found that the quality of a surface treated by the method of the invention is comparable with the quality of a honed surface and may even be better.
Advantageous embodiments are to be seen in the subordinate claims. The cutting tool can be single-edged or multiple-edged. Suitable cutting tools are, for example, an indexable cutting tip or a cutter spindle equipped with a plurality of indexable cutting tips. Coated bores, such as cylinder liners with a friction surface coating, are preferably reworked by dry spindle cutting. The tool with one or more cutters, such as a cutting spindle equipped with one or more indexable cutting tips, is introduced into the standing, internally coated cylinder liner. The cutting is performed without coolant or under minimal lubrication conditions. Vice versa, it is of course also possible for the workpiece to be driven while a fixed tool is used.
Suitable cutting materials are, for example, polycrystalline diamond, monocrystalline diamond, or a carbide coated with a vapor-deposited diamond layer.
Hard components can be contained in the structure of the tribological coating. These are, as a rule, hard, primary and eutectic silicon particles. The hard coating components at the surface can be exposed immediately after the dry machining.
Another advantageous embodiment provides for the use of a combination of dry machining and an additional procedure wherein after the dry machining the surface is textured in a one-step process by irradiation, especially with radiation grooving. Preferably a laser texturizing of the surface is performed. This laser texturizing can advantageously be limited in the case of cylinder friction surfaces to the area of the top dead center, that is, the point at which the direction of the movement of the piston reverses and its velocity is zero. The laser texturization creates pockets in the surface in which lubricant can collect later on during operation. This solution is a combination process combining the dry cutting operation with a subsequent operation for texturing the surface by means of a beam. The final surface in this case has both a dry-cut striated texture produced with a specifically shaped cutter or cutters, on which beam grooving is superimposed.
The coating to be treated is preferably a coating produced by plasma spraying methods. A preferred aluminum-silicon alloy is substantially copper-free, i.e., it contains less than 1% copper by weight.